Portable radios, such as portable telephones, are usually designed to hold the antenna at its retracted position in their housing on the move to make them as compact in size as possible.
With the antenna retracted in the radio housing, however, portable radios of this kind cannot accomplish satisfactory transmission and reception, and the sensitivity for receiving call signals is particularly low. Hence, it is necessary in the prior art to hold the antenna in its extended position while awaiting a call.
As a solution to this problem, there has been developed an antenna unit which permits transmission and reception even with a rod antenna unit retracted in the radio housing, as described in Japanese Pat. Laid-Open Gazette No. 245603/91.
This conventional antenna assembly will be described below with reference to FIGS. 10 to 12.
As shown in FIG. 10, the antenna assembly has its feeding metal pipe mounted on the top of a housing 101 of a portable radio 110 and its rod antenna element 103 adapted to be pushed into and drawn out of the radio housing 101 through a through hole bored in the feeding metal pipe 102.
The rod antenna element 103 carries at its tip a helical antenna element 104. Since the rod antenna element 103 and the helical antenna 104 each have a length equal to the quarter wavelength of a radio signal of a frequency f, a half-wave antenna is formed when the rod antenna element 103 is drawn out of the radio housing 101 to its extended position.
In this state, the lower end portion of the rod antenna element 103 makes electrical contact with the feeding metal pipe 102, which is connected via a feeder 105 to a transmitter-receiver circuit (not shown) on a circuit board 106.
When the rod antenna element 103 is retracted in the radio housing 101, the junction between the rod antenna element 103 and the helical antenna 104 makes contact with the feeding metal pipe 102 and the helical antenna element 104 projecting out of the radio housing 101 functions as a quarter-wave antenna.
Accordingly, even with the rod antenna element 103 retracted in the radio housing 101, the portable radio 110 can transmit and receive radio signals of the frequency f.
As described above, the conventional antenna unit permits transmission and reception even when the rod antenna element 103 is retracted in the radio housing 101, but the rod antenna element 103 is still held in contact with the feeding metal pipe 102; hence, when the portable radio 110 transmits, high-frequency signals also emanate from the rod antenna element 103 and cause noise, adversely affecting respective circuits in the radio housing 101.
Also when receiving with the rod antenna element 103 retracted in the radio housing 101, the wavelength of the helical antenna element 104 deviates from the quarter wavelength under the influence of the rod antenna element 103, impairing the sensitivity to incoming signals.
To avoid this, in the above prior art example the lower end portion 103 a of the rod antenna element 103, when retracted, is held in contact with a grounding segment 107 as shown in FIG. 13, by which the impedance viewed from the feeding metal pipe 102 is made infinity, preventing the rod antenna element 103 from functioning as an antenna.
To ground the lower end portion 103 a of the rod antenna element 103, however, it is necessary to provide an extra grounded circuit as well as the grounding segment 107. This inevitably introduces complexity in the antenna structure.
Further, since the rod antenna element 103, when retracted in the radio housing 101, is supported at its upper end by the feeding metal pipe 102 alone, the lower end portion 103a is unstable in position and fail to contact the grounding segment 107.
Moreover, with an increase in the frequency f of the radio signal for transmission and reception by the portable radio 110, the influence of R and L components of the feeding metal pipe 102 and other parts becomes increasingly nonnegligible. It is not an easy task to make the resonance frequency of the helical antenna element 104 match the frequency f of the radio signal by adjusting the length of the helical antenna element 104.
Additionally, even when the rod antenna element 103 is in its extended position as shown in FIG. 11, only one resonance frequency is available; hence, when the receive frequency and the transmit frequency differ as in the case of the portable telephone, high sensitivity for reception and large radiated power for transmission cannot be obtained.
Incidentally, there is also known antenna unit of the type that permits transmission and reception only by the helical antenna element 104 projecting out of the radio housing 101 but as is the case with the above-mentioned prior art example, this antenna unit does not provide two resonance frequencies either, and hence it fails to obtain high sensitivity for reception and large radiated power for transmission.